It is conventionally known that when shear force is added to a liquid in which fine particles are dispersed, it shows low viscosity under low shear force, but when the shear force to be added is increased, it shows a phenomenon of suddenly showing high viscosity, namely dilatancy. This dilatancy is a phenomenon caused by a temporal change in the packing condition of particles due to a sudden external force. When an assembly of spherical particles having the same size is considered for the sake of simplicity, theoretical value of void volume in the closest packing is 26%.
Under such a condition, the assembly of spherical particles can gently flow when a liquid sufficient for filling gaps among particles is absorbed. However, when a strong external force is suddenly added thereto, the assembly of spherical particles is transferred to a rough packing state. For example, since the void volume in the most rough packing is 48%, the aforementioned liquid is not sufficient even when the entire volume thereof is absorbed into the increased volume, thus entailing the presence of particles which are not soaked in the liquid but wear with one another.
This is the cause of the behavior similar to a brittle solid matter, effected in the dilatancy by the absorption of the liquid from the surface into inside and subsequent swelling of the volume and loss of the fluidity. An example of this is a phenomenon in which, when stepped on a seaside wet sandy soil with a foot, gaps among the sand particles are expanded and sea water is absorbed into sands so that the sandy soil looks like dry and becomes solid. The dilatancy phenomenon can also be observed when a precipitate of an inorganic crystals is filtered under reduced pressure.
“Dilatal” (registered trademark) manufactured by BASF, Germany, is known as a dilatant fluid. This is a product prepared by dispersing fine particles of an acrylic acid ester-styrene copolymer in water. Also, a fluid which shows dilatancy and comprises inorganic compound fine particles having a sphericity factor of 1.1 or less and a liquid medium has been reported in JP-A-8-281095, and the use of a silicone oil as the liquid medium is described in its claim.
Since the dispersion medium of conventionally known dilatant fluid is water or a volatile solvent, they have a problem in that the dispersing medium solidifies at low temperature and is apt to volatilize at high temperature. Even in case that the problem is not so serious, there is another problem in that they can be used only at around room temperature because of severe changes in viscosity. Also, though JP-A-8-281095 describes the use of “silicone oil” as the medium, there is no detailed description on the silicone oil, and there is no description on a particle dispersing agent in case that the silicone oil is used. Accordingly, they have a disadvantage in that, since the dispersed particles are hardly compatible with the medium silicone oil, the particles cannot be dispersed uniformly in a high blending ratio and are apt to cause precipitation thus entailing poor storage stability.